Nanofibrillar cellulose (NFC) consists of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. NFC is based on a natural polymer that is abundant in nature. Nanofibrillar cellulose has many potential uses for example based on its capability of forming viscous gel in water (hydrogel).
NFC production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. The concentration of NFC in dispersions is typically very low, usually around 1-5%. After the grinding process, the obtained NFC material is a dilute viscoelastic hydrogel. The material itself is usable as such in many applications, but logistic costs are too high to transport the material from the production site. In some applications, the high water content is not acceptable, i.e. the formulations do not tolerate large amounts of water.
Thus, there is an evident need for increasing the concentration of the final product so that the transport costs would be decreased and the NFC could be used in the final destination at a suitable concentration desired by the end user by simply redispersing the NFC in water.
Concentration or drying of NFC hydrogel is challenging, however. The specific surface area of NFC is very high due to its nanoscopic dimensions. Respectively, strong water retention is natural for NFC since water is bound on the surfaces of the fibers through numerous hydrogen bonds. Conventional separation techniques, such as filtration or evaporation are not feasible with NFC hydrogels, at least not on industrial level. The problem is widely recognized and heavily studied but not really solved.
The fundamental problem in mechanical water removal is the ability of NFC hydrogel to form a very dense and impermeable nanoscale membrane around itself, for example during filtration. The formed shell prevents diffusion of water from the gel structure, which leads to very slow concentration rates. The same applies to vacuum evaporation where the skin formation blocks the evaporation of water.
Another problem in drying of NFC is the non-redispersibility of the dried nanofibers. During the water removal, the NFC-water bonds are replaced with NFC-NFC interactions and the fibers are permanently aggregated. This can be prevented with the use of certain additives during the drying stage, such as CMC, or by chemical modification of the microfibril surface, e.g. oxidation or carboxymethylation. With those methods NFC can be re-activated after complete drying.
In the literature, the use of organic solvents in separation of MFC from water has been described. The proposed processes have been based on precipitation of dilute NFC dispersion into a non-solvent, such as isopropanol. Precipitation is typically carried out from dilute solutions with high speed mixing.
For example international publication WO0166600 describes a process where quaternary amine functionalized cellulose gel is added to isopropanol while stirring, at a rate of 1 gram 1% aqueous gel/2 ml isopropanol, whereafter the slurry is filtered through a synthetic straining cloth. After the filtration has gone about as far as it can, the wet filter cake is again dispersed in fresh amount of isopropanol at the same ratio, stirred, filtered and dried in an oven. Thus, the cake obtained after the first filtering step is still described as wet and must be treated once again with isopropanol.
According to European patent EP-0859011, a transparent viscous gel consisting of microfibrils of cationic cellulose is prepared, whereafter the drying can be performed by adding isopropanol or ethanol, or any other solvent having dewatering capability, to a 3% aqueous gel, whereafter the dehydrated microfibrils of cationic cellulose are recovered by filtration and dried in an oven. In this way a powder that maintains the rheological properties when redispersed in water is obtained. In Example 7 of this patent, a 3% viscous transparent gel of microfibrils of cationic cellulose is precipitated in isopropanol, filtered and dried in an oven to obtain dry product that recovers its original rheological properties upon addition of water.
In our experiments this protocol has been tested and found problematic. NFC is able to form colloidally stable dispersions also in alcohol media and filterability has not been improved as much as could be expected.
The article by Capadona J. R. et al. “A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates”, Nature Nanotech. 2, 765-769 (2007) describes gels made of nano-scale cellulose whiskers which are obtained through acid hydrolysis of tunicate mantles. The whiskers exist initially in aqueous dispersion and they are made to an organogel in a sol-gel process through solvent exchange with a water-miscible solvent, whereafter the gel is filled with a matrix polymer by immersing the gel in a solution of the polymer and dried. During the gel-forming step acetone was introduced on top of the aqueous whisker dispersion without mixing the layers. The acetone was exchanged daily and the acetone layer was gently agitated to promote the solvent exchange. After some days the acetone organogel was obtained, placed into a solution containing a polymer and dried after removal from the solution. The article also reports the use of acetonitrile, ethanol, methanol, isopropanol and tetrahydrofuran as solvents for making the organogel. The gel forming step through solvent exchange takes typically many days.